Image.resize() incorrectly centers non-nearest filters

[yromanenko]

(Discovered in the context of scipy, see scipy/scipy#4112 for more details)

Using an interpolation method other than 'nearest' with Image.resize results in an image shift away from (0,0) in the result (among other problems)

It behaves as though the filter kernel starts at (-0.5, -0.5), but is walked across the image without a corresponding change in step size.

Repeated application of Image.resize with Image.BILINEAR results in the image shrinking and walking off:

even more weird, but still walking with Image.BICUBIC:

while 'Image.NEAREST' behaves as expected:

I started looking at Geometry.c, and it seems like it needs a bit of love, but I am very unfamiliar with the code so I'm hesitant to start rewriting things :-)

Here's the test code:

from PIL import Image

src = Image.open("src.png").convert('F')
cbc = src
lnr = src
nrs = src

for idx in xrange(0, 128):
    cbc = cbc.resize(tuple(2 * i for i in cbc.size), Image.BICUBIC)
    cbc = cbc.resize(src.size, Image.BICUBIC)

    lnr = lnr.resize(tuple(2 * i for i in lnr.size), Image.BILINEAR)
    lnr = lnr.resize(src.size, Image.BILINEAR)

    nrs = nrs.resize(tuple(2 * i for i in nrs.size), Image.NEAREST)
    nrs = nrs.resize(src.size, Image.NEAREST)

    nrs.convert('L').save("nrs_%03d.png" % (idx))
    lnr.convert('L').save("lnr_%03d.png" % (idx))
    cbc.convert('L').save("cbc_%03d.png" % (idx))

Thanks

Platform information:

Python 2.7.8 (default, Jul 13 2014, 17:11:32)
[GCC 4.2.1 Compatible Apple LLVM 5.1 (clang-503.0.40)] on darwin
Type "help", "copyright", "credits" or "license" for more information.

[homm]

Thanks for reporting. I've also noticed this issue recently. There is no need to fix this particular issue, because im.resize() should be whole replaced with im,stretch() internal implementation. These and many other errors will go away.

[wiredfool]

That bicubic one is pretty freaky.

[hugovk]

Just for fun, here's the output on another image.

Bilinear: http://giphy.com/gifs/TlK63EPFRDPqETTakec

Bicubic: http://giphy.com/gifs/TlK63EvgnEChMti41FK